The present invention relates to measurement of the distance between substantially parallel metal tubes forming a bundle and the thickness of an oxide layer which may possibly cover the tubes. It finds a particularly important, although not exclusive, application in the measurement of distance between the metal tubes forming the sheaths of rods belonging to a nuclear fuel assembly and the thickness of oxide layers covering the tubes.
When the fuel assemblies removed from the core of a reactor after an operating cycle are not completely burnt up, they are reloaded into the core at another position. Before such reloading, the assemblies must be checked so as to make sure that they can be run again in the reactor without excessive risk of sheath failure.
It is in particular necessary to check that the spacing between adjacent rods is not reduced locally in an excessive way. The reduction of spacing may in particular be caused by bending of certain rods, locally reducing the spacing between mutually facing surfaces of the sheaths. The reduction may generate hot points and result in local failure of the sheaths, especially when they contact each other. But even if the consequences are not as serious, the variations of spacing create defects in the cooling flow and disturb the thermodynamic cycle detrimentally to the correct operation of the reactor.
It is also desirable to measure the thickness of the oxide layer formed on the surface of the sheaths in contact with the coolant. The oxide layer reduces the heat exchange coefficient between the sheath and the coolant and consequently causes overheating of the sheath. In fuel assemblies for pressurized water nuclear reactors, the zirconium base alloy sheaths are progressively covered with a layer of zirconia. When the thickness of this layer exceeds a limit, about 100 .mu.m in present day pressurized water reactors, oxide formation tends to increase and may lead to cracking of the sheath and to the egress of fission products into the coolant.
At the present time, the spacing between the rods of an assembly is generally measured by causing a camera to travel opposite the gap between rods, along the assembly. The camera is equipped with a reference grid and an operator follows the variations of spacing between the rods on the display screen. This method of measurement is tiring and its reliability is uncertain.
It is known to measure the oxide thickness at given points of a rod sheath situated at the periphery of a fuel assembly using an eddy current probe which is brought into contact with the sheath to be examined, by moving it orthogonally to the sheath, then raising it and bringing it in front of the next measurement point. The device for such measurement has no other function. The method of bringing the probe to each point of measurement requires precise location of the position of the sheath so that the probe be placed radially. The measurements are very slow when it is decided to make the measurement at sufficiently numerous and closely spaced points.
European 0,170,341 discloses eddy current measurement of a film thickness, with reference to a calibration curve prepared from reference plates; there is no suggestion to measure the distance between parallel tubes. Japanese patent application No. 58166203 only teaches that a similar method makes it possible to measure the thickness of a layer on a tube. As to U.S. Pat. No. 4,814,703, it discloses a device for measuring the variations of thickness of an epoxy layer.